By virtue of asymmetric distributions of ionic channels and pump mechanisms in apical and basolateral membranes of epithelial cells, epithelial tissues of renal and other origin regulate the composition of body fluids. In order to obtain a better understanding of the mechanisms involved, methods of fluctuation or noise analysis are to be used that allow the electrodiffusive Na+ channel densities and single channel Na+ currents to be measured at the apical membranes of the cells. In this way, the possible mechanisms whereby ionic permeability is regulated will be investigated with special emphasis on the phenomenon of "autoregulation" whereby Na+ channel density appears to change in compensatory fashion in response to procedures that inhibit Na+ entry into the cells. In complementary fashion, methods of patch clamp will be used to study the apical membranes of renal tubule cells to obtain direct information of the unitary currents and their kinetics under various physiological conditions (mineralocorticoids, ADH) known to alter Na+ absorption and K+ secretion.